Generally, it is known in the art that a constant depression (C.D.) carburetor, in principle, seeks to achieve five goals; that is, high fuel metering accuracy, elimination or at least substantial reduction in the lag in change in fuel flow rates at transient conditions, elimination of the problems attendant the changing of the fuel metering function from one to another metering orifice as is the usual case in fixed metering orifice-fixed venturi carburetors, the use of a basically simple single metering orifice for providing the entire range of required metered fuel flows and the attainment of the power capacity of staged fixed orifice carburetors by means of only a single induction passage or barrel.
The prior art C.D. carburetors, however, exhibit some disadvantages. For example, in the prior art C.D. carburetors of the slide-piston metering rod and diaphragm type the relatively large size and weight of the piston-diaphragm (or stepped piston) device present, among other problems, the problem of inertia. Also, the high dimensional accuracies of such piston devices result in high manufacturing costs. Further, the prior art C.D. carburetors employing such slide pistons are confronted with problems of friction. That is, the slide piston is usually subjected to a transverse pressure differential resulting in a sideways force being exerted on the piston which, in turn, causes frictional forces and a related hysteresis. The thusly generated hysteresis, in turn, results in slightly differing axial positions of the piston, for a given rate of air flow, depending upon whether the piston is moving toward a position of greater rate of metered fuel flow or a position of reduced rate of metered fuel flow.
The prior art has attempted to solve the problems of the slide piston type C.D. carburetor. That is, the prior art has suggested that the slide piston should be replaced as by an air baffle, variable venturi arrangement or by means of a second upstream throttle valve with such being coupled to the metering rod and a vacuum piston or diaphragm as by means of related linkages. It was believed that such, because of their ability to be held by journals or pivots, would result in far less friction, under actual operation, than the slide piston.
However, such attempts by the prior art have not proven to be successful. That is, generally, the friction reducing advantages of such elements, resulting from having them mounted in or suspended by bearings becomes lost due to the complicated connection thereof to the metering rod and the related pressure responsive diaphragm. That is, such prior art attempts have resulted in the employment of shafts, bellcranks, connecting rods and multiple bearings in order to achieve coupling of the throttle to the metering rod and to the pressure responsive diaphragm by way of tortuous friction-creating circuitous paths. Further, such connecting means of the prior art devices have to pass through as well as between different pressure regions thereby making the use of friction creating seals necessary.
In those C.D. carburetors of the prior art employing what may be considered as simple interiorly disposed linkage means between the C.D. throttle and metering rod, two different solutions are employed for operatively connecting the third member of the trinity of the elements of the C.D. carburetor, namely, the C.D. diaphragm or piston means, for conjoint operation. The first of such two solutions was to use complicated and heavy externally situated linkages between the C.D. throttle and the C.D. diaphragm or piston. However, such a connection to the C.D. diaphragm or piston still results in the undesired friction caused by the many attendant bearings and seals. The second of such two solutions was to eliminate the C.D. piston or diaphragm and to replace the function thereof with an unbalanced eccentrically suspended C.D. throttle. In such devices the suction or vacuum created downstream of the unbalanced C.D. throttle produced the effect of a separate piston directly on the C.D. throttle by, in effect loading one side of the throttle more than the other side. However, such an attempt by the prior art has not been successful. For example, the effective vacuum or suction-subjected area of such unbalanced throttles diminishes with increased opening thereof thereby resulting in difficulties in operation where increased throttle opening is required as well as being unable to maintain a sufficient degree of constant depression characteristics over the required range of operation. Another important difficulty arises from the fact that the unbalanced throttle is highly susceptible to the pulsations of the air flow. With C.D. diaphragm carburetors such air flow pulsation is a very small, if at all significant, problem. In those C.D. carburetors having a throttle connection to the C.D. diaphragm, the problem of air flow pulsations is inherently reduced to not more than a tolerable amount. However, with the unbalanced C.D. throttle of the prior art, no pneumatic damping or smoothing of air flow pulsations is possible.
The invention as herein disclosed and claimed is primarily directed to the solution of the aforestated as well as other related and attendant problems.